fix: work around inability to build
fix: work around inability to build

Seeing an issue like this:

    https://github.com/commercialhaskell/stack/issues/157

Fix from here:

    https://stackoverflow.com/a/39019781

With this, this (ie. building updated Ferret docs) works:

    git clone https://github.com/wincent/docvim.git
    cd docvim
    pacman -S stack happy
    bin/docvim \
        -c ~/.vim/pack/bundle/opt/ferret \
        ~/.vim/pack/bundle/opt/ferret/README.md \
        ~/.vim/pack/bundle/opt/ferret/doc/ferret.txt

wincent/docvim